1. Field of Invention
The present invention relates to devices for testing semiconductor wafers and more particularly to a novel probe card for semiconductor wafer testing.
2. Background
Integrated circuits are made in a bulk parallel process by patterning and processing semiconductor wafers. Each wafer contains many identical copies of the same integrated circuit referred to as a “die.” Semiconductor wafers must be tested before the die is cut into individual integrated circuits and packaged for sale. If defects are detected the defective die can be culled before wasting resources packaging a defective part.
To test a wafer, a probe card is commonly used which comes into contact with the surface of the wafer. The probe card generally contains three unique characteristics: (1) an XY array of individual probes that bend in the Z direction to allow contact with the die; (2) an electrical interface to connect the card to a circuit test apparatus; and (3) a rigid reference plane defined in such a way that the probe card can be accurately mounted in the same location on the wafer tester. FIG. 14 illustrates an array of generic probes (A) on a substrate (B). Probe tips (C) for each probe in the array (A) are allowed to bend in the Z direction (perpendicular to the substrate (B)). When the probe card is brought in contact with the die, the Z-direction bending allows for a solid contact with the probe tip (C). The probe card ultimately provides an electrical interface that allows a circuit test apparatus to be temporarily connected to the individual die. This method of die testing is extremely efficient because many die can be tested at the same time. To drive this efficiency even higher, probe card manufactures are making larger probe cards with an ever-increasing numbers of probes.
Probe cards have been manufactured in a variety of ways over the years. Historically, these cards were made by gluing or by manually clamping small wires to a rigid frame. This style of probe card was generally limited to large contact pads at widely spaced intervals. The probes could be manually manipulated to achieve their proper positional locations in X, Y, and Z. Another method of producing accurately located probes is to drill a substrate material with an array of holes and place the probes into the hole array.
As the semiconductor industry has continued to decrease the size of integrated circuits (and consequently the die size), both the contact pad size and their spacing have decreased accordingly. Thus, new manufacturing methods have utilized photolithographic and micro-machining techniques to very accurately position the probes within the needed tolerances and to pack more probes onto a single card. Current state of the art semiconductor manufacturing routinely produces contact pad sizes of 80 um with inter-pad spacing on a 100 um pitch. A current probe card may have as many as 5,000 or more individual probes that must accurately engage the contact pads. The XYZ positional accuracy required for each of the individual probes is on the order of ±15 um in all directions.
Semiconductor wafers are processed in such a way that the wafer surface is extremely flat and when these wafers are held in place for testing the probe card must present the probe tips so that all the probe tips contact the wafer at roughly the same time as the probe card is lowered onto the wafer. This is accomplished by achieving what is known as “tip planarity” on the probe card. The probe card is manufactured so that the individual probe tips are all equally spaced from the probe card reference surface. Generally the Z-axis position of the probes is determined by using a probe card analyzer to contact the probe card to a flat metal surface that is electrically grounded. The probe card is moved toward this surface while the electrical state of each probe is monitored. When a probe comes into contact with the metal surface the position of the probe card is recorded. This method is the industry standard for determining probe card planarity.
Determining the XY location of the probe is a much more complicated matter. A quick way to look for misalignment of a probe from its intended position is to place the probe card under a standard microscope and look for probes that look “out of place.” The Z position is very difficult to determine using this method due the depth of focus for most microscope objectives. The XY location of each probe is far easier to see, but it is still very difficult to gauge how far each probe is from its correct location given the small dimensions. This inspection method can only find probes that are grossly out of XY alignment. Probes are generally spring structures with some flexibility in the plane of the probe substrate. As such, they can inadvertently bend away from their ideal intended orientation by normal use, misuse or damage.
To determine alignment, a technician generally begins by visually inspecting the probe cards and if the technician can see a problem with the unaided eye then there is a severe problem and the card must be pulled off the line to repair the problem. However, when the technician does not see a misalignment with the naked eye, he must make the decision as to whether to pull the probe card off the line. Of course the technicians can always opt to send the probe card to secondary inspection. This, however, is very labor intensive and costly—both in inspection costs and testing line downtimes. Also, the equipment needed for secondary inspection may not be available where the probe cards are being inspected, further aggravating costs and downtime. If the technician decides not to send the probe into secondary inspection, then a misaligned probe card may give false positives and reduce the testing line yield. In either scenario, faulty probe card inspection will reduce yield and increase costs of the testing process.
Several methods have been proposed to increase the accuracy of locating the exact XY probe position (and thus identifying misalignment) quickly and inexpensively. For example, U.S. Pat. No. 6,023,172, entitled “Light-Based Method And Apparatus For Maintaining Probe Cards” uses two intersecting light beams to define the proper location stability issues. It also requires a precision optical alignment system to determine proper alignment.
U.S. Pat. No. 4,918,374, entitled “Method And Apparatus For Inspecting Integrated Circuit Probe Cards” utilizes a computer-controlled table that moves in the X and Y directions. The table has various conductive areas. The probe card is dragged across the table so that the probe card probes make an electrical contact with the table. Misalignments are identified based on the conductive areas with which the probe card is making contact. This method requires a complicated and expensive apparatus.
U.S. Pat. No. 5,657,394 entitled “Integrated Circuit Probe Card Inspection System” and U.S. Pat. No. 6,118,894 entitled “Integrated Circuit Probe Card Inspection System” use a video camera that is mounted on a XY translation stage to determine the actual location of the probe tip. These systems implement video processing algorithms to determine the XYZ position of each probe and can be quite accurate. This method very accurately determines the XY position of the probe but requires a very expensive probe card analyzer and can take up to several hours to get results. This method has the further drawback that debris on the probe tips can preclude the video processing system from resolving the true location of the centroid, which is required to begin individual probe inspection.
U.S. Pat. No. 6,933,738 entitled “Fiducial Alignment Marks On Microelectronic Spring Contacts” uses an alignment mark that is patterned during the same step as the feature to be aligned, thus the mark is fixed relative to the probe tip, and is further designed to be lower than the tip surface. The lower position makes the alignment mark less vulnerable to debris buildup. The probe cards implementing these marks are then scanned using conventional video algorithms. Thus, this method still has the drawbacks of video processing—i.e., they are expensive and very time consuming.
What is needed, therefore, is a method and apparatus that allows for quick and inexpensive inspection to identify misalignments on a probe card.